[36]In the Rabbit and probably other Monodelphous Mammalia the segmentation is nearly though not quite regular.[37]VideF. M. Balfour, “Comparison of the early stages of development in Vertebrates.”Quart. Jour. of Micr. Science, July, 1875.[38]VideRemak,Entwicklung d. Wirbelthiere; and Götte,Entwicklung d. Unke.[39]Van Beneden,“Développement embryonnaire des Mammifères.”Bull. de l’Acad. Belgique, 1874.[40]Phil. Trans.1875.[41]Fol,Archives de Zoologie Expérimentale,Vol.IV.1875.[42]Flemming,“Entwick. der Najaden,”Sitz. d. Akad. Wiss. Wien,Bd.4, 1875.[43]Kowalevsky,Mem. Akad. Petersburg, Series VII, 1871.[44]Archiv. f. mikr. Anat.Vol.XIII.1877.[45]Metschnikoff,Zeitschrift f. wiss. Zoologie, 1874.[46]VideSchultze,Archiv. f. mikr. Anat.Vol.XI.; and F. M. Balfour,Monograph on the Development of Elasmobranch Fishes.[47]VideKlein,Quart. Journal of Micr. Science, April, 1876. Bambeke,Mem. Cour. Acad. Belgique, 1875. His,Zeit. für Anat. u. Entwicklung.Vol.I.[48]VideBambeke,loc. cit.[49]VideBobretzky,Zeitschrift für wiss. Zoologie,Vol.XXIV., 1874.[50]Loc. cit.[51]Bulletins de l’Acad. Belgique,Tom.XXIX., 1870.[52]Though less obvious in the ovum of the fowl than in that of some other types, they may nevertheless be demonstrated there without very much difficulty.[53]Quart. Journ. of Micr. Science,Vol.XV.pp.39, 40.[54]Quart. Journ. of Micr. Science,Vol.XVIII.p.41.[55]At the time when my observations on Elasmobranchii were carried out, this peculiar condition of the nucleus had not been elucidated.[56]For this term as well as for the terms telolecithal and centrolecithal I am indebted to Mr Lankester.[57]Mayer,Jenaische Zeitschrift,Vol.XI.[58]Ed. van Beneden,Bull. d. l’Acad. roy. Belgique,2mesérie,Tom.XXVIII.No.7, 1869,p.54.[59]Zeitschrift für wiss. Zool.,Vol.XXIV.1874.[60]Bobretzky,Zeit. f. wiss. Zool.,Bd.XXXI.1878.[61]Metschnikoff,“Embry. Stud. Insecten,”Zeit. für wiss. Zool.,Bd.XVI.1866. My own observations on this form accord in the main with those of Metschnikoff.[62]VideWeismann,Entwicklung d. Dipteren; and Auerbach,Organologische Studien.[63]VideLudwig,Zeit. f. wiss. Zool., 1876.[64]and[65]Ed. van Beneden,Bull. Acad. Belgique,Vol.XXVIII.1869.
[36]In the Rabbit and probably other Monodelphous Mammalia the segmentation is nearly though not quite regular.
[37]VideF. M. Balfour, “Comparison of the early stages of development in Vertebrates.”Quart. Jour. of Micr. Science, July, 1875.
[38]VideRemak,Entwicklung d. Wirbelthiere; and Götte,Entwicklung d. Unke.
[39]Van Beneden,“Développement embryonnaire des Mammifères.”Bull. de l’Acad. Belgique, 1874.
[40]Phil. Trans.1875.
[41]Fol,Archives de Zoologie Expérimentale,Vol.IV.1875.
[42]Flemming,“Entwick. der Najaden,”Sitz. d. Akad. Wiss. Wien,Bd.4, 1875.
[43]Kowalevsky,Mem. Akad. Petersburg, Series VII, 1871.
[44]Archiv. f. mikr. Anat.Vol.XIII.1877.
[45]Metschnikoff,Zeitschrift f. wiss. Zoologie, 1874.
[46]VideSchultze,Archiv. f. mikr. Anat.Vol.XI.; and F. M. Balfour,Monograph on the Development of Elasmobranch Fishes.
[47]VideKlein,Quart. Journal of Micr. Science, April, 1876. Bambeke,Mem. Cour. Acad. Belgique, 1875. His,Zeit. für Anat. u. Entwicklung.Vol.I.
[48]VideBambeke,loc. cit.
[49]VideBobretzky,Zeitschrift für wiss. Zoologie,Vol.XXIV., 1874.
[50]Loc. cit.
[51]Bulletins de l’Acad. Belgique,Tom.XXIX., 1870.
[52]Though less obvious in the ovum of the fowl than in that of some other types, they may nevertheless be demonstrated there without very much difficulty.
[53]Quart. Journ. of Micr. Science,Vol.XV.pp.39, 40.
[54]Quart. Journ. of Micr. Science,Vol.XVIII.p.41.
[55]At the time when my observations on Elasmobranchii were carried out, this peculiar condition of the nucleus had not been elucidated.
[56]For this term as well as for the terms telolecithal and centrolecithal I am indebted to Mr Lankester.
[57]Mayer,Jenaische Zeitschrift,Vol.XI.
[58]Ed. van Beneden,Bull. d. l’Acad. roy. Belgique,2mesérie,Tom.XXVIII.No.7, 1869,p.54.
[59]Zeitschrift für wiss. Zool.,Vol.XXIV.1874.
[60]Bobretzky,Zeit. f. wiss. Zool.,Bd.XXXI.1878.
[61]Metschnikoff,“Embry. Stud. Insecten,”Zeit. für wiss. Zool.,Bd.XVI.1866. My own observations on this form accord in the main with those of Metschnikoff.
[62]VideWeismann,Entwicklung d. Dipteren; and Auerbach,Organologische Studien.
[63]VideLudwig,Zeit. f. wiss. Zool., 1876.
[64]and[65]Ed. van Beneden,Bull. Acad. Belgique,Vol.XXVIII.1869.
SYSTEMATIC EMBRYOLOGY.
SYSTEMATIC EMBRYOLOGY.
Introduction.
In all the Metazoa the segmentation is followed by a series of changes which result in the grouping of the embryonic cells into definite layers, or membranes, known as the germinal layers. There are always two of these layers, known as the epiblast and hypoblast; and in the majority of instances a third layer, known as the mesoblast, becomes interposed between them. It is by the further differentiation of the germinal layers that the organs of the adult become built up. Owing to this it is usual, in the language of Embryology, to speak of the organs as derived from such or such a germinal layer.
At the close of the section of this work devoted to systematic embryology, there is a discussion of the difficult questions which arise as to the complete or partial homology of these layers throughout the Metazoa, and as to the meaning to be attached to the various processes by which they take their origin; but a few words as to the general fate of the layers, and the general nature of the processes by which they are formed, will not be out of place here.
Of the three layers the epiblast and hypoblast are to be regarded as the primary. The epiblast is essentially the primitive integument, and constitutes the protective and sensory layer. It gives rise to the skin, cuticle, nervous system, and organs of special sense. The hypoblast is essentially the digestive and secretory layer, and gives rise to the epithelium lining the alimentary tract and the glands connected with it.
The mesoblast is only found in a fully developed condition in the forms more highly organized than the Cœlenterata. It gives origin to the general connective tissue, internal skeleton, the muscular system, the lining of the body cavity, the vascular, and excretory systems. It probably in the first instance originated from differentiations of the two primary layers, and in all groups with a well-developed body cavity it is divided into two strata. One of them forms part of the body wall and is known as the somatic mesoblast, the other forms part of the wall of the viscera and is known as the splanchnic mesoblast.
Diagram of a Gastrula.Fig. 55. Diagram of a Gastrula.(From Gegenbaur.)a.blastopore;b.archenteron;c.hypoblast;d.epiblast.
Fig. 55. Diagram of a Gastrula.(From Gegenbaur.)
a.blastopore;b.archenteron;c.hypoblast;d.epiblast.
A very large number not to say the majority of organs are derived from parts of two of the germinal layers. Many glands for instance have a lining of hypoblast which is coated by a mesoblastic layer.
The processes by which the germinal layers take their origin are largely influenced by the character of the segmentation, which, as was shewn in the last chapter, is mainly dependent on the distribution of the food-yolk. When the segmentation is regular, and results in the formation of a blastosphere, the epiblast and hypoblast are usually differentiated from the uniform cells forming the wall of the blastosphere in one of the two following ways.
(1) One-half of the blastosphere may be pushed in towards the other half. A two-layered hemisphere is thus established which soon elongates, while its opening narrows to a small pore (fig. 55). The embryonic form produced by this process is known as a gastrula. The process by which it originates is known as embolic invagination, or shortly invagination. Of the two layers of which it is formed the inner one (c) is known as the hypoblast and the outer (d) as the epiblast, while the pore leading into its cavity lined by the hypoblast is the blastopore (a). The cavity itself is the archenteron (b).
(2) The cells of the blastosphere may divide themselves by a process of concentric splitting into two layers (fig. 56, 3). The two layers are as before the epiblast and hypoblast, and theprocess by which they originate is known as delamination. The central cavity or archenteron (F) is in the case of delamination the original segmentation cavity; and not an entirely new cavity as in the case of invagination. By the perforation of the closed two-walled vesicle resulting from delamination an embryonic form is produced which cannot be distinguished in structure from the gastrula produced by invagination (fig. 56, 4). The opening (M) in this case is not however known as the blastopore but as the mouth.
Formation of a GastrulaFig. 56. Diagram shewing the formation of a Gastrula by delamination.(From Lankester.)Fig. 1. Ovum.Fig. 2. Stage in segmentation.Fig. 3. Commencement of delamination after the appearance of a central cavity.Fig. 4. Delamination completed, mouth forming atM.In fig. 1, 2 and 3Ec.is ectoplasm, andEn.is entoplasm.In fig. 4Ec.is epiblast andEn.hypoblast.
Fig. 56. Diagram shewing the formation of a Gastrula by delamination.(From Lankester.)
Fig. 1. Ovum.Fig. 2. Stage in segmentation.Fig. 3. Commencement of delamination after the appearance of a central cavity.Fig. 4. Delamination completed, mouth forming atM.In fig. 1, 2 and 3Ec.is ectoplasm, andEn.is entoplasm.In fig. 4Ec.is epiblast andEn.hypoblast.
Section of ovum of EuaxesFig. 57. Transverse section through the ovum of Euaxes during an early stage of development.(After Kowalevsky.)ep.epiblast;ms.mesoblastic band;hy.hypoblast.
Fig. 57. Transverse section through the ovum of Euaxes during an early stage of development.(After Kowalevsky.)
ep.epiblast;ms.mesoblastic band;hy.hypoblast.
When segmentation does not take place on the regular type the processes above described are as a rule somewhat modified. The yolk is usually concentrated in the cells which would, in the case of a simple gastrula, be invaginated. As a consequence of this, these cells become (1) distinctly marked off from the epiblast cells during the segmentation; and (2) very much more bulky than the epiblast cells. The bulkiness of thehypoblast cells necessitates a modification of the normal process of embolic invagination, and causes another process to be substituted for it,viz.the growth of the epiblast cells as a thin layer over the hypoblast. This process (fig. 57) is known as epibolic invagination. The point where the complete enclosure of the hypoblast cells is effected is known as the blastopore. All intermediate conditions between epibolic and embolic invagination have been found.
Two stages in the development of Stephanomia pictumFig. 58. Two stages in the development of Stephanomia pictum.(After Metschnikoff.)A. Stage after the delamination.ep.epiblastic invagination to form pneumatocyst.B. Later stage after the formation of the gastric cavity in the solid hypoblast.po.polypite;t.tentacle;pp.pneumatophore;ep.epiblastic invagination to form pneumatocyst;hy.hypoblast surrounding pneumatocyst.
Fig. 58. Two stages in the development of Stephanomia pictum.(After Metschnikoff.)
A. Stage after the delamination.ep.epiblastic invagination to form pneumatocyst.B. Later stage after the formation of the gastric cavity in the solid hypoblast.po.polypite;t.tentacle;pp.pneumatophore;ep.epiblastic invagination to form pneumatocyst;hy.hypoblast surrounding pneumatocyst.
In delamination, when the segmentation is not uniform, or when a solid morula is formed, the differentiation of the epiblast and hypoblast is effected by the separation of the central solid mass of cells from the peripheral cells (fig. 58A).
In the case of epibolic invagination as well as in that of the type of delamination just spoken of, the archenteric cavity is in most cases secondarily formed in the solid mass of hypoblast (fig. 58B).
In ova with a partial segmentation there is usually some modification of the epibolic gastrula.
Many varieties are found in the animal kingdom of the types of invagination and delamination just characterized, and in not a few forms the layers originate in a manner which cannot be brought into connection with either of these processes.
Epibolic Gastrula of BonelliaFig. 59. Epibolic Gastrula of Bonellia.(After Spengel.)A. Stage when the four hypoblast cells are nearly enclosed.B. Stage after the formation of the mesoblast has commenced by an infolding of the lips of the blastopore.ep.epiblast;me.mesoblast;bl.blastopore.
Fig. 59. Epibolic Gastrula of Bonellia.(After Spengel.)
A. Stage when the four hypoblast cells are nearly enclosed.B. Stage after the formation of the mesoblast has commenced by an infolding of the lips of the blastopore.
ep.epiblast;me.mesoblast;bl.blastopore.
The mesoblast usually originates subsequently to the two primary layers. It then springs from one or both of the other layers, but its modes of origin are so various that it would be useless to attempt to classify them here. In cases of invagination it often arises at the lips of the blastopore (fig. 57and 59), and in other cases part of it springs as paired hollow outgrowths of the walls of the archenteron. Such outgrowths are shewn infig. 60, B and C atpv. The cavity of the outgrowths forms the body cavity, and the walls of the outgrowths the somatic and splanchnic layers of mesoblast (fig. C.sp.andso.). The archenteron is in part always converted into a section of the permanent alimentary tract and the section of the alimentary tract so derived is known as the mesenteron. There are however usually two additional parts of the alimentary tract, known asthestomodaeumandproctodaeum, derived from epiblastic invaginations. They give rise respectively to the oral and anal extremities of the alimentary tract.
Three stages in the development of SagittaFig. 60. Three stages in the development of Sagitta.(A and C after Bütschli and B after Kowalevsky.) The three embryos are represented in the same positions.A. Represents the gastrula stage.B. Represents a succeeding stage in which the primitive archenteron is commencing to be divided into three parts, the two lateral of which are destined to form the mesoblast.C. Represents a later stage in which the mouth involution (m) has become continuous with alimentary tract, and the blastopore has become closed.m.mouth;al.alimentary canal;ae.archenteron;bl.p.blastopore;pv.perivisceral cavity;sp.splanchnic mesoblast;so.somatic mesoblast;ge.generative organs.
Fig. 60. Three stages in the development of Sagitta.(A and C after Bütschli and B after Kowalevsky.) The three embryos are represented in the same positions.
A. Represents the gastrula stage.B. Represents a succeeding stage in which the primitive archenteron is commencing to be divided into three parts, the two lateral of which are destined to form the mesoblast.C. Represents a later stage in which the mouth involution (m) has become continuous with alimentary tract, and the blastopore has become closed.
m.mouth;al.alimentary canal;ae.archenteron;bl.p.blastopore;pv.perivisceral cavity;sp.splanchnic mesoblast;so.somatic mesoblast;ge.generative organs.
Bibliography.
(107)K. E. von Baer.“Ueb. Entwicklungsgeschichte d. Thiere.” Königsberg,1828‑1837.(108)C. Claus.Grundzüge d. Zoologie.Marburg und Leipzig,1879.(109)C. Gegenbaur.Grundriss d. vergleichenden Anatomie.Leipzig, 1878.Videalso Translation.Elements of Comparative Anatomy.Macmillan andCo., 1878.(110)E. Haeckel.Studien z. Gastræa-Theorie. Jena, 1877, and alsoJenaische Zeitschrift,Vols.VIII.andIX.(111)E. Haeckel.Schöpfungsgeschichte.Leipzig.Videalso Translation.The History of Creation.King andCo., London, 1876.(112)E. Haeckel.Anthropogenie.Leipzig.Videalso Translation.Anthropogeny(Translation). Kegan Paul andCo., London, 1878.(113)Th. H. Huxley.The Anatomy of Invertebrated Animals.Churchill, 1877.(114)E. R. Lankester. “Notes on Embryology and Classification.”Quart. J. of. Micr. Science,Vol.XVII.1877.(115)A. S. P. Packard.Life Histories of Animals, including Man, or Outlines of Comparative Embryology.Holt andCo., New York, 1876.(116)H. Rathke.Abhandlungen z. Bildung und Entwicklungsgesch. d. Menschen u. d. Thiere.Leipzig, 1833.
Dicyemidæ.
The structure and development of these remarkable parasites in the renal organs of the Cephalopoda have recently been greatly elucidated by the researches of E. van Beneden; and although a male element has not been discovered, yet the embryos originate from bodies which have a close similarity to ordinary ova.
Van Beneden has shewn that Dicyema consists in the adult state of (1) a single layer of ciliated epiblast cells, somewhat modified anteriorly to form a cephalic enlargement; and of (2) one large nucleated hypoblast cell enclosed within the epiblast. There are two kinds of embryo, both developed from germs which originate in the hypoblast cell. The two kinds of embryo arise in individuals of somewhat different forms. The one kind, called by Van Beneden the vermiform embryo, arises in the more elongated and thinner examples of Dicyema which have been named Nematogens. These embryos pass directly into the parent form without metamorphosis.
The second kind of embryo, called infusoriform, is very different from the parent, and has a free existence. Its eventual history is not known. It originates in the shorter and thicker individuals of Dicyema; which have been called Rhombogens.
The Vermiform Embryos.The germs or cells which give rise to the vermiform embryos originate endogenously in the protoplasmic reticulum of the axial hypoblast cell. They appear as small but well-defined spheres, with a minute body in thecentre. In these spheres a cortical layer becomes differentiated, which gradually increases in thickness and gives rise to the body of a cell, the nucleus and nucleolus of which are respectively formed from the inner part of the original sphere and the minute central body. These germs can originate in all parts of the hypoblast cell and are frequently very numerous.
Dicyema typusFig. 61. A. Gastrula stage of Dicyema typus. B. Veriform embryo of Dicyema typus. (From Gegenbaur, after E. van Beneden.)
Fig. 61. A. Gastrula stage of Dicyema typus. B. Veriform embryo of Dicyema typus. (From Gegenbaur, after E. van Beneden.)
The germ when completely formed undergoes a segmentation very similar to that of an ordinary ovum. It divides first into two and then into four approximately equal segments. Of the four segments one, however, remains passive for the remainder of the development. The other three divide and arrange themselves so as partially to enclose in a cup-like fashion the passive cell (fig. 61A). The six cells resulting from their division again divide, giving rise to twelve cells, which nearly enclose the passive cell, leaving only a small aperture at one point. The whole process by which the central cell becomes enclosed is, as E. van Beneden points out, identical with a gastrula formation by epibole, and the space where the central cell is left uncovered is the blastopore. The central cell itself gives origin to the hypoblast cell of the adult, and the peripheral cells to the epiblast.
By this time the embryo has assumed an oval form, and the blastopore is situated at the pole of the long axis of the oval where the cephalic enlargement is eventually formed.
The subsequent development consists mainly in the closure of the blastopore, and an increase in the number of the epiblast cells. Before the development is completed, and while the embryo is still in the body of the parent, two germs, destined themselves to give rise to fresh embryos, appear in the hypoblast cell, one on each side of the nucleus (fig. 61B). The embryo continues to elongate, while the anterior cells become converted into the polar cells. Cilia appear simultaneously over the general surface, and the embryo makes its way out of the body of the parent, usually at the cephalic pole, and becomes itself parasitic in the renal organ of the host in which it finds itself.At the time of birth the embryo may contain a number of germs and sometimes even developing embryos.
Infusoriform Embryos.The infusoriform embryos are capable of living in sea-water and almost certainly lead a free existence. In their most fully developed condition so far known they have the following rather complicated structure (fig. 62D, E, F, G).
The body is somewhat pyriform, with a blunt extremity which is directed forwards in swimming, and a more pointed extremity directed backwards. The former may be spoken of as the anterior, and the latter as the posterior extremity or tail. At the anterior extremity are situated a pair of refractive bodies (r) which lie above an unpaired organ which may be called the urn.
The structure of the urn, the refractive bodies, and the tail may be dealt with in succession.
The urn consists of three parts: (1) a wall (u), (2) a lid (l), and (3) contents (gr). The wall of the urn is hemispherical in form, and composed of two halves in apposition (fig. F). Its concavity is directed forwards, and in its edge are imbedded a number of rod-like corpuscles which appear as a ring near the surface in a full-face view (fig. D). The lid has the form of a low pyramid with its apex directed outwards. It is made up of four segments (fig. D). The contents of the urn, which completely fill up its cavity, are four polynuclear cells arranged in the form of a cross which appear with low powers as granular bodies (fig. F). They are frequently ejected, apparently at the will of the embryo.
The refractive bodies (r), two in number, one on each side of the middle line, are composed of a material which is not of a fatty nature, and which is passive to the majority of reagents. Each is enveloped in a special capsule, and at times more than one refractive body is present in each capsule. The tail is a conical structure formed of ciliated granular cells.
No plausible guess has been made as to the function either of the urn or of the refractive bodies.
The infusoriform embryos originate from germs, which have however a different origin to the germs of the vermiform embryos. One to five cells appear in the axial hypoblast cell, ina way not clearly made out, and each of them gives rise by an endogenous process to several generations of cells, all of which develop into infusoriform embryos.
Infusoriform embryo of DicyemaFig. 62. Infusoriform embryo of Dicyema.A. B. C. Three of the later stages in the development.D. E. F. Three different views of the full-grown larva. D. from the front, E. from the side, and F. from above.G. side view of urn.u.wall of urn;l.lid of urn;r.refractive bodies;gr.granular bodies filling the interior of the urn.
Fig. 62. Infusoriform embryo of Dicyema.
A. B. C. Three of the later stages in the development.D. E. F. Three different views of the full-grown larva. D. from the front, E. from the side, and F. from above.G. side view of urn.
u.wall of urn;l.lid of urn;r.refractive bodies;gr.granular bodies filling the interior of the urn.
The primitive cell is called by Van Beneden a Germogen. In its protoplasm a number of germs first appear endogenously, but the nucleus of the germogen does not assist in their formation. They eventually become detached from the parent cell, around which they are concentrically arranged. A second and then a third generation of germs are formed in the same way, till the whole of the protoplasm of the primitive cell is absorbed in the formation of these germs, and nothing of it remains but the nucleus. The germs so formed are arranged in about three concentric layers, of which the innermost is the youngest. One to five masses of germs may be present in a single Rhombogen. The germs undergo a division, in the course of which their nuclei exhibit very beautifully a spindle modification. In the course of the segmentation the embryo gradually assumes its permanent form, and four of the cells composing it can be distinguished from the remainder by their greater size (fig. 62A,u). The two largest of these give rise to the wall of the urn, and also give origin to four smaller cells (fig. 62B,gr) which eventually become polynuclear and constitute the four granular cells in the urn. The two other cells become the lid of the urn. The partsof the urn lie at first side by side, but in the course of development the cells which form the wall of the urn travel inwards, and the four granular cells are carried into their concavity. At the same time the cells which form the lid of the urn alter their position so as to overlie the wall of the urn. The two cells immediately above the urn give rise to the refractive bodies (fig. 62A, B, C,r) and the remainder of the cells of the embryo become the tail (fig. 62C). The embryo becomes ciliated, and attains its nearly full development before leaving the parental tissues. It usually passes out at the cephalic extremity.
As has already been stated, it is probable that the infusoriform embryos leave the renal organs of their host and lead a free existence. What becomes of them afterwards is not however known, though there can be little doubt that they serve to carry the species to new hosts.
Till the further development of the infusoriform embryo is known it is not possible to arrive at a definite conclusion as to the affinities of this strange parasite. Van Beneden is anxious to form it, on account of its simple organization, into a group between the Protozoa and the Metazoa. It appears however very possible that the simplicity of its organization is the result of a parasitic existence; a view which receives confirmation from the common occurrence of the process of endogenous cell formation in the axial hypoblast cell. It has been clearly shewn by Strasburger that endogenous cell formation is secondarily derived from cell division; so that the occurrence of this process in Dicyema probably indicates that the hypoblast was primitively multicellular. It is not improbable that the enigmatical infusoriform embryo may develop into a sexual form, the progeny of which are destined to complete the cycle of development by becoming again parasitic in the renal organ of a Cephalopod.
Bibliography.
(117)E. van Beneden.“Recherches sur les Dicyemides.”Bull. d. l’Académie roy. de Belgique,2esér. T.XLI.No.6 andT.XLII.No.7, 1876.Videthis paper for a full account of the literature.(118)A. Kölliker.Ueber Dicyema paradoxum den Schmarotzer der Venenanhänge der Cephalopoden.(119)Aug. Krohn.“Ueb. d. Vorkommen von Entozoen, etc.”Froriep Notizen,VII.1839.
Orthonectidæ.
A number of minute parasites infesting various Nemertines, Turbellarians, and Ophiuroids have recently been studied by Giard and Metschnikoff, the former of whom has placed them in a special group which he calls the Orthonectidæ. They were first discovered by W. C. McIntosh.
In the adult state they are[66](Metschnikoff) somewhat pear-shaped bodies formed of a kind of plasmodium of cells with irregular lobate processes. In the interior of this body are eggs in all stages of development. In the type observed by Metschnikoff (Intoshia gigas) the ova undergo a regular segmentation, resulting in the formation of a blastosphere in which an inner layer is subsequently formed by delamination. A smaller and a larger kind of embryo are formed; but all the embryos in each female belong to one type. The larger become females and the smaller males.
The female embryos are ovoid. The outer layer of cells or epiblast becomes ciliated, and divided into nine segments, of which the second is marked off from the remainder by the absence of cilia, and by being provided with refractive corpuscles. The inner layer which surrounds a central cavity, and might be supposed to be the hypoblast, becomes according to Metschnikoff converted into ova.
The male embryos are more elongated than the female, from which they further differ in only having six segments. The cells of the inner layer eventually divide up into spermatozoa.
The larvæ probably become free, and while in the free state impregnation would appear to be effected. When the female larvæ become parasitic they undergo a metamorphosis, the stages of which have not been observed; but in the course of which the epiblast cells probably unite into a plasmodium.
The observations of Giard are in several points irreconcilable with those of Metschnikoff, but from the statements of the latter it appears possible that Giard has made two genera from the males and females of one species; and that Giard’s account of an unequal segmentation followed by an epibolic gastrula, in one of his species, has arisen from two segmenting ova temporarily fusing together. Giard has given a description of internal gemmiparous reproduction, upon the accuracy of which doubts have been thrown by Metschnikoff. The affinities of the Orthonectidæ are as obscure as those of the Dicyemidæ; though there can be but little doubt that their organization has been much simplified in correlation with their parasitic habits. The origin of the genital products in the axial tissue is a feature they have in common with the Dicyemidæ.
Bibliography.
(120)Alf. Giard.“Les Orthonectida classe nouv. d. Phylum des Vers.”Journal de l’Anat. et de la Physiol.,Vol.XV.1879.(121)El. Metschnikoff.“Zur Naturgeschichte d. Orthonectidæ.”Zoologischer Anzeiger,No.40‑43, 1879.
[Ch. Julin.“Rech. sur l’organization et le devel. d’Orthonectides.”Arch. Biol.Vol.III.1882.
E. Metschnikoff.“Untersuchungen üb. Orthonectidæ.”Zeit. f. Wiss. Zoologie,Vol.XXXV.1881.
For general account of Orthonectidæ,videSpengel.Biolog. Centralblatt,No.6.]
[66]This at any rate holds true for the type investigated by Metschnikoff. The full history of other forms is not yet known.
[66]This at any rate holds true for the type investigated by Metschnikoff. The full history of other forms is not yet known.
Although within the last few years greater advances have probably been made in our knowledge of the development of the Porifera than of any other group, yet there is much that is still very obscure, and it is not possible to make general statements applying to the whole group.
Calcispongiæ.The form which has so far been most completely worked out isSycandra raphanus, one of the Calcispongiæ (Metschnikoff, Nos.132and134, F. E. Schulze, Nos.139and142), and I shall commence my account with the life history of this species.
The ovum in Sycandra as in other Spongida has the form of a naked amœboid nucleated mass of protoplasm. From the analogy of the other members of the group, there is no doubt that it is fertilized by a male spermatic element, though this has not as yet been shewn to be the case—and the changes which accompany fertilization are quite unknown.
Segmentation of Sycandra raphanusFig. 63. Successive stages in the segmentation of Sycandra raphanus.(Copied from F. E. Schulze.)A. stage with eight segments still arranged in pairs, from above.B. side view of stage with eight segments.C. side view of stage with sixteen segments.D. side view of stage with forty-eight segments.E. view from above of stage with forty-eight segments.F. side view of embryo in the blastosphere stage, eight of the granular cells which give rise to the epiblast of the adult are present at the lower pole.cs.segmentation cavity;ec.granular cells which form the epiblast;en.clear cells which form the hypoblast.
Fig. 63. Successive stages in the segmentation of Sycandra raphanus.(Copied from F. E. Schulze.)
A. stage with eight segments still arranged in pairs, from above.B. side view of stage with eight segments.C. side view of stage with sixteen segments.D. side view of stage with forty-eight segments.E. view from above of stage with forty-eight segments.F. side view of embryo in the blastosphere stage, eight of the granular cells which give rise to the epiblast of the adult are present at the lower pole.
cs.segmentation cavity;ec.granular cells which form the epiblast;en.clear cells which form the hypoblast.
The segmentation and early stages of development take place in the tissues of the parent. The segmentation is somewhat peculiar, though a modification of a regular segmentation. The ovum divides along a vertical plane, first into two, and then into four equal segments. But even when two segments are formed, each of them has one end pointed and the other broader. The pointed ends give rise to the ciliated cells of the future larva, and the broad ends to the granular cells. Instead of the next division taking place, as is usually the case, in a horizontal (equatorial) plane, it is actually effected along two vertical planesintermediate in position between the two first planes of segmentation. Eight equal segments are thus formed, each of which has the form of a pyramid. All the segments are situated in a single tier, and are so arranged as to give to the whole ovum the form of a flat cone, the apex of which is formed by the pointed extremities of the constituent segments (fig. 63B). The apices of the segments do not however quite meet, but they leave a central space, which is an actual perforation (fig. 63A) through the axis of the ovum, open at both ends. The first indications of this perforation appear when only four segments are present, and it is to be regarded as the homologue of the segmentation cavity of other ova. The next plane of division is horizontal (equatorial), and the apices of the eight cells are segmented off as a tier of small cells. At the completion of this division (fig. 63C), the ovum is formed of sixteen cells arranged in two superimposed tiers. The ovum now assumes somewhat the form of a biconvex lens, in the axis of which the central perforation is stillpresent. At the close of the next stage, forty-eight cells are present arranged in four tiers (fig. 63D and E), the two outer tiers containing eight cells each, and the two inner sixteen. The two inner tiers probably arise by the simultaneous appearance of two equatorial furrows dividing the original tiers into two, and by the subsequent simple division of the cells of the two inner of the tiers so formed. At the close of the stage the eight basal cells become granular (fig. 63F). At the same time the central part of the segmentation cavity becomes enlarged, while its terminal apertures become narrowed and finally, shortly after the end of this stage, closed. The axial perforation thus acquires the character of a closed segmentation cavity. While the ovum itself becomes at the same time a blastosphere.